In the art of range determination systems and, more particularly, the art of auto focus cameras, it is often desirable that radiation being received from a remote object pass through the taking lens of the camera where it is split into two portions. The first, and usually major portion of the radiation, passes to the film or to the electronics of the video camera; while the other portion of the light is directed to the auto focus circuitry to produce a signal indicative of the focus condition of the camera lens. Systems of this type may be seen, for example, in Stauffer U.S. Pat. No. 4,185,191, issued Jan. 22, 1980.
It is common in the prior art systems to employ a partially silvered mirror in the optical path so as to direct about twenty percent of the radiation to the auto focus circuitry, while passing about eighty percent of the radiation to the image plane of the camera. It is desirable to maximize the amount of radiation that passes through to the camera image plane and accordingly, efforts have been made in the past to increase the efficiency of the auto focus detectors by shaping them so as to occupy as much of the image of radiation at the detection plane as possible. As is seen, for example, in Stauffer U.S. Pat. No. 4,230,942, issued Oct. 28, 1980 and assigned to the assignee of the present invention, the detectors may have their corners clipped to approximate a semicircle and the radiation image may be distorted to form an oval pattern. Shaping the detectors to fit within the radiation image or altering the image to extend the area thereof has, however, required additional effort and equipment and has created problems with respect to positioning and adjustment because the radiation image and the detector pairs have to be adjusted extremely carefully in order to maximize the radiation used. Because of this difficulty, a compromise is usually reached where the size of the radiation image is increased so that the detector pairs can more easily fit within the confines thereof, but in doing so, additional radiation is wasted resulting in less radiation being available for exposing the camera film or video camera circuitry.
Finally, when utilizing zoom lenses, for example, in motion picture or video cameras, a problem arises due to the fact that the partially silvered mirror must be placed between the afocal zoom portion of the lens, and the master lens. The length of the partially-silvered mirror unnecessarily increases the length of the zoom lens in an undesirable fashion.